bornavirus

:  a single-stranded RNA virus (family Bornaviridae) that has a spherical virion with a lipoprotein envelope and chiefly infects the central nervous system of birds and mammals (such as sheep, dogs, and horses) causing mild to fatal disease Bornaviruses, a group of RNA viruses that cause disease in horses and sheep, first inserted their genetic material into ancestral human DNA at least 40 million years ago …—Tina Hesman Saey, Science News, 30 Jan. 2010